Previous shaft angle encoders suffer in regards to accuracy as a result of eccentricity errors, i.e., errors in alignment of the optical slits on the code wheel and phase plate in respect to the shaft. In one previous encoder, radially opposed detectors described in U.S. Pat. No. 4,108,539 detect the amount of the eccentricity error and a calculator corrects the encoding of the shaft angle. However, the detection and correction technique requires an additional detector as well as the calculation and correction of the encoded angle by a calculator. Therefore, it is desirable to reduce the sensitivity of a comparator of an encoder to eccentricity errors rather than to detect and correct for this error. The described encoder has a low sensitivity to eccentricity errors as a result of the close spacing between the channels. This close spacing has been achieved by a special design of the lenses. Particularly, the detector lens has three bifurcated and truncated lenses. Each of these lenses is made of two siamesed and truncated elements having closely spaced optical axes. Thus, lenses which have a size adequate to insure good efficiency and have closely spaced optical axes are obtained which results in the encoder having a low sensitivity to eccentricity errors.